1. Field of the Invention
The present invention relates to an optical information reading apparatus that irradiates light to a reading object such as a bar code to read the image thereof through light reflected thereby. More particularly, the present invention relates to an optical information reading apparatus that has a dust-proof plate for preventing dust from penetrating into a case from a reading opening.
2. Description of Related Art
It is known that light emitted by a light emitting device such as a light emitting diode or the like is irradiated onto a bar code attached to goods. Light reflected from the bar code is focused on a predetermined reading position through an image forming optical system. A light receiving device such as a CCD (charge-coupled device) positioned at the reading position reads the image of the bar code.
Normally, in this kind of apparatus, light is irradiated onto a reading object such as a bar code from a reading opening formed on the case of the apparatus, and light reflected from the bar code is guided into the case from the reading opening to form the image of the bar code on the light receiving device. At a place positioned a little inward from the reading opening, there is provided a dust-proof plate to prevent the light emitting device, the light receiving device, and the image forming optical system from being subjected to dust. The dust-proof plate transmits the light emitted by the light emitting device therethrough. Thus, the light emitted by the light emitting device passes through the dust-proof plate, thus being irradiated to the bar code or the like located outside the case. Light reflected from the bar code enters into the case and passes through the dust-proof plate. The image forming optical system forms the image of the bar code on the light receiving device.
However, because the light emitted by the light emitting device is irradiated onto the reading object through the dust-proof plate, the light is partly reflected by the dust-proof plate. Incidence of the light reflected from the dust-proof plate on the image forming optical system prevents accurate reading of the image of the bar code, thus causing optical noise for the light reflected from the bar code or the like.